It is desirable in many types of couplings to locate the field coil radially inwardly of both the inductor and rotor. It is further desirable to use a stationary mounting of the field coil on the housing of the coupling to avoid the slip rings and brushes often associated with rotating coils. The reliability of brush operation is dependent on clean atmospheric conditions, sufficient humidity and proper mechanical tolerences if good electrical contact and minimum wear during rotation is to be maintained. As shown in U.S. Pat. No. 2,817,029, the inductor drum and rotor are typically formed as a concentrically mounted cup-within-a-cup to permit the stationary mounting of the field coil. The coil is typically supported by a simple bracket. However, this bracket does not provide for adequate heat dissipation from the coil.
The design configuration shown in the aforementioned patent uses a largely rotating flux path to return flux to the rotor poles and drum. This path includes the rotor support and output shaft. Accordingly, it may be noted that these parts serve both a mechanical and magnetic function, lending design economy to the configuration.
Alternatively, a largely non-rotating flux return path is known in the art, as shown in U.S. Pat. No. 3,051,859. In this patent, an enlarged coil-supporting bracket lies in the flux-return path. Improved heat dissipation is thereby provided because of greater coil contact area with the bracket, as well as improved heat conductivity in the bracket itself. The improved heat dissipation, however, is provided at a cost. Because the use of the shaft and/or rotor support as flux carrying members is lost, the axial length of the coupling device must be increased to provide the requisite total area for the flux path.